Re: Disabling the freewheling diode

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "claude masetto" <claudmas-at-optusnet-dot-com.au> 

Surely this schottkey diode in series with the mosfet conducts at the same
time that the mosfet does so why isn't it rated for the same current. Am I
missing something.
Claude
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, August 30, 2003 3:54 AM
Subject: RE: Disabling the freewheling diode


 > Original poster: "Jan Wagner" <jwagner-at-cc.hut.fi>
 >
 > Hi!
 >
 >  > In a SSTC H-bridge, the internal freewheeling diode of the
 >  > MOSFET is disabled by a clever arrangement of external
 >  > diodes. A schottkey diode above the drain keeps the internal
 >  > diode from getting forward biased, so that an ultrafast
 >  > recovery external diode can provide the path of any reverse
 >  > EMF from the load. Any current passing trough the MOSFET must
 >  > pass trough the schottkey diode.
 >  >
 >  > In a typical design, I see MOSFETS capable of handling 20A
 >  > continous, with a 3 A schottkey diode on top.
 >
 > (the diode can be at the source or drain pin, symmetric...)
 >
 >  > Why is the schottkey diode not dimentioned for a similar 20A?
 >
 > Because luckily it conducts only for a short period. If the SSTC is
in-tune
 > and the drive signals have ~50% duty, the diode ideally
 > conducts no current at all. When the SSTC is really grossly out of tune
 > i.e. pri current draw and the drive voltage are highly out
 > of phase, almost no current will be flowing into the primary, and also the
 > diode conducts very little current. When slightly out of
 > tune, the diode will conduct the (sine shaped) current close to where the
 > sine waveform crosses zero, so, again, it's not very hard
 > on the diodes as the average current is quite low.
 >
 > Only when you're doing PWM, i.e. changing the duty cycle of the drive
 > signals so that the duty t.ex. averages at 25%, the diodes
 > will conduct a large current - in the worst case, the peak primary current
 > - and have to be rated the same as the mosfets (or, hmm,
 > probably only half the current rating of mosfets... (?))
 >
 > cheers,
 >   - Jan
 >
 > --
 > ********************************************************
 >   high voltage at http://www.hut.fi/~jwagner/tesla/
 >   Jan OH2GHR - GSM +358-41-4682893
 >   jwagner-at-cc.hut.fi - Jan.Wagner-at-cern.ch
 >
 >